With the progress of small-size, high-speed and lightweight electronic devices such as cellular phones and personal computers, the capacitor used for these electronic devices is demanded to have a smaller size, a larger capacitance and a lower ESR.
Among these capacitors, a tantalum capacitor has a large capacitance for its size and also has good performance and therefore, this capacitor is preferably used.
In order to increase the capacitance of a tantalum capacitor, it is necessary to increase the amount of the powder used or use a sintered body increased in the surface area by finely pulverizing the tantalum powder.
In the method of increasing the amount of the powder used, a large-size capacitor structure inevitably results and the demand for reduction in the size and weight cannot be satisfied. Also, in the method of increasing the surface area by finely pulverizing the tantalum powder, the pore diameter of the tantalum sintered body decreases and a high-capacitance tantalum capacitor cannot be produced or ESR cannot be decreased.
In order to overcome these defects, a capacitor obtained from a sintered body using a powder material having a higher dielectric constant than that of tantalum and having a low density has been proposed. Niobium is attracting attention as the material having a high dielectric constant.
A niobium capacitor is produced in the same manner as a tantalum capacitor.
A sintered body of niobium powder is generally used as the anode material of the niobium capacitor. For example, a niobium powder is granulated by mixing fine niobium powder with a liquid binder and then shaped by compression shaping and after implanting an anode lead therein, the shaped article is sintered at a high temperature in a high vacuum to produce an electrode called a niobium anode sintered body. The surface of this niobium anode sintered body is electrolytically oxidized (electro-chemically formed) to produce an electrically non-conducting insulating layer (insulating oxide layer of niobium), a counter electrode layer (cathode layer) such as manganese dioxide and electrically conducting polymer is formed on the electrically non-conducting insulating layer, a carbon paste, a silver paste and the like are sequentially stacked thereon, and the entirety is jacket-molded with a material such as epoxy resin, whereby a niobium capacitor is fabricated.
The niobium capacitor is inferior in the stability as compared with the tantalum capacitor.
More specifically, the insulating oxide layer of tantalum, which is produced by electrolytic oxidation, is formed of only tantalum pentoxide and has very high stability. On the other hand, niobium forms an insulating oxide layer comprising mainly niobium pentoxide containing a stable semiconducting suboxide (e.g., niobium dioxide, niobium monoxide) and therefore, the electrical stability is poor, which appears, for example, in increase of leakage current value or decrease of breakdown voltage (ratio of electrochemical voltage/operation voltage).
Furthermore, the increase in the thickness of tantalum pentoxide at the electrochemical formation is about 2 nm/V, whereas the increase in the thickness of niobium pentoxide at the electrochemical formation is as large as about 3.7 nm/V. Therefore, the insulating oxide layer comprising mainly niobium pentoxide is weak against mechanical or thermal stimulation and is readily broken. Particularly, when an electrically conducting polymer is used for the counter electrode, the increase of leakage current at the jacket-molding using a material such as epoxy resin, and the increase of leakage current caused by solder reflow at mounting on the substrates come out as a serious problem. In recent years, lead-free soldering is increasingly demanded and the elevation of solder reflow temperature to cope with this demand causes more increase in the leakage current value of niobium capacitor. In this way, because of its low breakdown voltage, the niobium capacitor is limited in its use to fields where the operation voltage is low and the leakage current does not affect.
In order to overcome these problems, various approaches have been proposed.
JP-A-10-242004 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) describes a technique of partially nitriding niobium powder to improve the leakage current value of a niobium capacitor.
WO00/49633 and WO01/6525 describe a technique of limiting the total content of specific elements of Fe, Ni, Co, Si, Na, K and Mg to 350 ppm or less, and thereby improving the fluctuation in the leakage current value of a niobium capacitor.
WO00/15555 and WO00/15556 describe niobium oxide powder reduced in oxygen, which is used for a capacitor and obtained by partially reducing niobium oxide in the presence of an oxygen getter metal. These publications disclose the properties of the anode but do not describe a production example of a capacitor.
WO02/15208 and WO02/93596 describe a technique of incorporating or alloying other elements into niobium or niobium monoxide powder, and thereby decreasing the leakage current value of a capacitor. These publications do not describe the breakdown voltage and the soldering heat resistance.
WO01/35428 describes a technique of coating the niobium powder surface with elements Al, Si, Ti, Zr, Mo, W, Y and Ta and incorporating metals of Al, Si, Ti, Zr, Mo, W, Y and Ta into the niobium oxide barrier layer. This publication discloses the properties of the anode produced from this niobium powder but does not describe a production example of a capacitor.
WO01/26123 describes a composition where a part or the majority of niobium surface is covered with a compound having a silicon-oxygen bond or with a silicon-containing compound capable of producing a silicon-oxygen bond through hydrolysis, condensation, oxidation, thermal reaction or the like. This publication discloses that the compound having a silicon-oxygen bond is taken into the oxide dielectric film at the electrochemical formation and the leakage current value and deterioration in capacitance of niobium capacitor are reduced, but does not refer to the breakdown voltage and the soldering heat resistance.
In this way, various attempts have been made, but the improvement is not yet satisfactory and enhancement of breakdown voltage and soldering heat resistance is particularly demanded.